1. Field of the Invention
The present invention relates to a method for jam-resistant transmission of speech signals, processed via vocoders, by radio, utilizing spread frequency bands (SSMA) or a rapid radio frequency change (frequency hopping FH).
2. Description of the Prior Art
For the use of tactical radio apparatus in the military field, a high-resistance against intentional jamming of all types is of particular significance. To this end, use must be made of transmission methods whose transmission signals cannot be readily impaired by strong jamming. Such a transmission method is, for example, made available for the so-called spread spectrum multiple access (SSMA) modulation technique. An application of this method is disclosed, for example, in the German Letters Pat. No. 2,121,117, fully incorporated herein by this reference. The useful signal to be transmitted is widely spread in its frequency band by an identification modulation, and this spread is canceled (or nullified) again at the receiving side by a correlator. Strong jammers in the form of individual spectral lines are thereby considerably attenuated with respect to the useful signal. An additional method is to make use of a rapid radio frequency change (frequency hopping) during transmission so that a strong jamming transmitter has difficulty adjusting rapidly enough to the radio frequency change just employed. Radio systems making use of a radio frequency hopping procedure are known, for example, from U.S. Pat. No. 4,037,159. The jamming resistance of signals to be transmitted is greater, the smaller the useful (or effective) band width of the signal can be dimensioned in comparison with the transmission band width. For this reason, it is advantageous, in the case of speech transmission, in order to increase the jamming resistance, in addition to the application of the described method, to make use of vocoders. The speech signal is here evaluated (or analyzed) on the transmitting side by a vocoder analyzer with respect to its specific speech parameters, and only the latter are transmitted in coded form towards the receiving side. A the receiving side, in a vocoder synthesizer, the original speech signal is synthesized by means of these parameters in conjunction with a pulse-shaped and a noise-shaped excitation function. In addition to a reduction of the useful band width, the coded form of the signal to be transmitted is simultaneously desirable for an encoding with a high degree of secrecy which, in the case of military applications, in addition to the previously-cited jamming resistance, as a rule is likewise required.
As practice shows, the described methods for jam-resistant communication transmission occasionally gives rise to inherent jammings, In the case of using the SSMA technique, a great number of stations simultaneously use the same frequency band. For a specific receiver, which, with the aid of the correlator allocated thereto, merely receives one signal--determined by the identification modulation--as the useful signal, all simultaneously-transmitted useful signals for the other stations act as spurious (or jamming) signals, or noise signals, respectively, at the receiver input. In other words, relative to the specified frequency band, the number of stations simultaneously in operation must be restricted to such an extent that a receiver can still satisfactorily receive the signal assigned thereto from the noise. The same applies in the case of application of a rapid frequency hopping operation. Here, it is necessary to proceed on the assumption that altogether only a restricted number of radio frequencies on the order of magnitude of approximately 500 are available. Of these 500 radio frequencies, a selection of approximately 120 radio frequency channels can then be made available, for example, to one or more radio circuits. A superior control of several radio circuits in order to realize a synchronous hopping method is virtually not possible for many reasons. For reasons of frequency availability it is likewise impossible to assign to each radio circuit a reserved channel bundle. Since the radio circuits run synchronously neither in the jump phase nor in the jump frequency program, therefore, with calculatable probability, the result must be overlappings of variable width of the information blocks of the same frequency radiated by the various radio apparatus of various radio circuits.
In the case of the mentioned redundancy-decreasing transmission of speech by way of a vocoder, in practice, an acceptable speech quality results if fewer than 2% of all transmission blocks are jammed, in case the block length is sufficiently small and the speech synthesizer is informed of the jamming. In the case of a group of 128 radio hopping frequencies, the jamming quantity would already be exceeded in the case of simultaneous operation of more than three radio circuits within the jamming field intensity range.